The field of the invention is chemistry, electrical and wave energy wherein non-metals in an aqueous bath are coated on a metal base.
The present invention is particularly related to a coating bath for the cataphoretic coating of metallic surfaces, which contains aqueous solutions and/or aqueous dispersions of salts of cationic film-forming agents. Furthermore, the invention concerns the production of the coating bath and its use for the preparation of coatings by cataphoresis, in order to impart to the coatings an improved adhesion and to ensure an improved corrosion protection for the coated metallic surfaces.
The state of the art of cataphoretic coating of metallic surfaces and the bath compositions used therein may be ascertained by reference to U.S. Pat. Nos. 3,230,162 and 3,455,806; U.S. Pat. Application Ser. No. 193,591 of Robert Jerabek et al, filed Oct. 28, 1971; and West German Published Applications 2,320,301 and 2,357,075; the disclosures of which are incorporated herein.
In general, electrically conductive workpieces, preferably of iron or other metals, are subjected to a pretreatment before the anaphoretic or cataphoretic coating step. This pretreatment is, in most cases, a complicated multistage process for the purpose of providing the surface to be coated with sufficient corrosion protection and for taking care of a sufficiently firm adhesion of the coating thus deposited. Normally, iron surfaces receive as the pretreatment a phosphating process with zinc phosphate, zinc calcium phosphate, or iron phosphate. Due to the complicated control of the various phosphating methods, such a pretreatment is susceptible to disturbances and has the disadvantage of strong fluctuations with respect to maintaining the coating weight, the structure of the phosphating layer, etc. With respect to optimum operating conditions, the concentrations of the layer-forming substances, the accelerators, the temperature, and the pH value are criteria which can be accurately maintained only with great expenditure of time and money.
Other possible pretreatment methods, such as for example, the chromatizing method, moreover make it difficult to accurately predict the attainable corrosion protection and adhesion.
Although during the phosphating dip difficultly accessible parts of a workpiece are also wetted by the phosphating solution, so that a layer formation can be effected also at these locations, an entrainment of foreign ions is particularly great in this process due to strongly scooping parts, so that the subsequent electrophoretic dip coating bath is thereby contaminated and a considerable reduction in the quality of the coating can occur.
It is known in the method of spray-phosphating that there is no film formation at all, or only an incomplete film formation at difficulty accessible places, such as, for example, in cavities, due to the type of process involved.
It is an object of the present invention to overcome the aforedescribed disadvantages inherent in the normally customary pretreatment and optionally entirely omit the pretreatment by phosphating, without impairing the corrosion protection effect of the electrophoretic dip coating.